Molecular Design Of D-A-π-A Carbazole Dye Sensitizers With Different π-bridge And Acceptor: DFT And TD-DFT Investigation

In pursuit of cleaner and more economical renewable energy,many scholars focused on the dye-sensitized solar cells(DSSCs).DSSCs have the advantages of low cost,strong variability,easy manufacture,and high efficiency.Comparing to organic metal dyes,non-metallic organic dyes have been considered as the best substitutes for ruthenium dyes,because of their high molar absorption coefficients,simple synthetic methods,environmentally friendly,and low cost.Based on the previous researches,carbazole derivatives were selected as electron donors,benzothiadiazole was introduced as an additional receptor,thienyl or bipythiophene or trisphenylene or furyl or phenyl groups as π-bridge,cyanoacrylic acid or carboxyl groups instead of cyanoacetic acid carboxyl groups or carboxyl groups of o-nitrobenzoic acid instead of cyanoacrylic acid as acceptors,five groups(fifteen)of "D-A-π-A" carbazole dyes were designed.The ground state geometries,frontier molecular orbitals(FMOs),absorption spectra,and excitation states of these dyes were investigated by using density functional theory(DFT)and time-dependent DFT(TD-DFT).The molecular structure of the dye sensitizers was compared with the relationship between photoelectric properties.With the N-butylcarbazole as the electron donor,benzothiadiazole as an additional receptor,thienyl or bipythiophene or trisphenylene or furyl or phenyl groups as π-bridge,cyanoacrylic acid as acceptors,five carbazole dyes with different π-bridges were designed.The results show the dyes with thiophene or furan as the π-bridges have better coplanarity than dyes with benzene as π-bridges,the better coplanarity is benefit to the transmission of electrones.All designed carbazole dyes have the appropriate HOMO and LUMO levels,which can receive the excitation of sunlight and can injection of electrons into the Ti O2 conduction band.With the longer of the π-bridge,the HOMO level is higher,the Eg is smaller,and the capacity of the hole transport is stronger.The frontier molecular orbit show that the HOMO of all dye molecules are mainly delocalized on the carbazole donor and benzothiadiazole,and the LUMO are mainly contributed by the part of "A-π-A".The HOMO and LUMO of all the TYZ-1~5 have effective overlaps on the π-bridge,and the HOMO and LUMO show the π and π* characters,respectively.Therefore,the electrons excited from the HOMO to LUMO and this excitation belongs to LE when absorbed lights.This result is confirmed by the absorption calculation based on the TD-DFT.The absorption result also shows that the excitation from HOMO to LUMO has the mainly contribution larger than 95.5%.The PCEs of all the dyes except for the TYZ-5 are all larger than 5.0%.All the first excited geometries of the carbazole dyes have better planarity than these of ground state.The reasonable SS from 51.9 to 98.1 nm of all the five dyes reduce the possibility of self-absorption at a great extent.Therefore,all the carbazole dyes,especially the TYZ-2 and TYZ-3 have photo-sensitiveness,and can be used as promising candidates in DSSCs as well.Based on the structure of TYZ-1,TYZ-2,TYZ-3,TYZ-4,and TYZ-5,five groups of "D-A-π-A" dyes were designed by replacing the carboxyl groups with cyano groups and o-nitrobenzoic acid as new acceptors.The strongest UV-visible absorption peaks of all dyes were between 550 and 650 nm.The dyes with onitrobenzoic acid as a receptor in the group I~IV have the largest absorption wavelength,this maybe because the introduction of the phenyl group lengthebed the conjugation chains,and the molecular absorbance redshifts.The maximum absorption wavelength of TYZ-5-2 in group five is 560.40 nm,which is a little shorter than that of the TYZ-5-1(567.06 nm).The blue shift of TYZ-5-2 maybe due to the larger dihedral angle bwteen the two benzene rings.Therefore,the dye TYZ-3-2(λabs=648.76 nm,λemi=743.56 nm,PCE=5.8%)with the o-nitrobenzoic acid in the receptor and have better photoelectric properties.